This invention discloses a catalytic converter for a diesel engine. More specifically, it discloses a catalytic converter that traps out the particulates in the exhaust gas and retains the particulates so that they can be burned off intermittently. Then the catalytic converter is again loaded with trapped particulates that are burned off at the end of the next cycle.
Catalytic converters are now standard equipment on gasoline powered automobiles, and their practicality for gasoline engines is well demonstrated. Catalytic converters for diesel engines pose different problems which have not yet been solved. Diesel exhaust is cooler than the exhaust from a gasoline engine, especially when the diesel engine is idling or running at low power output. Sometimes the diesel exhaust is so cool that a catalytic converter cannot light off and burn the easily-combustible carbon monoxide and hydrocarbons in the exhaust. Even when the diesel engine is running at high power output, the exhaust is seldom hot enough to burn the particulates therein. The particulates would pass through the converter and add to the suspended solids in the atmosphere. It is especially important to remove these diesel particulates because they are suspected of being carcinogenic.
The problem of diesel particulates and previous attempts to remove them have been described in three reports sponsored by the U.S. Environmental Protection Agency. These are EPA-600/7-79-232b, EPA-625/9-79-004, and "Ignition Properties and Catalytic Oxidation of Diesel Particulate," of the Battelle Columbus Laboratories, 1982. These reports describe particulate traps that comprised quartz wool, stainless steel wool, and ceramic honeycombs similar to those used for catalyst supports in automobile converters. The quartz wool was not nearly rugged enough. The stainless steel collapsed when it was heated by the burning particulates. The honeycomb tested at Battelle was a bundle of square channels having thin porous walls through which the exhaust gas was made to pass. The face of the honeycomb was a square grid, like a checkerboard, but with very small squares, say, 200 squares per square inch. If all the red squares in the supposed checkerboard are plugged on the inlet face of the honeycomb, and all of the black squares are plugged on the outlet face, then the gas must pass through one ceramic wall during its passage through the honeycomb. This arrangement did trap out the particulates, within on upon the porous walls of the honeycomb. But the pressure drop rose rapidly and reached six inches of mercury after only fifteen minutes. Such a pressure drop is much too high to be practical. The collected particulates were catalyzed for burning by spraying them with a solution of a copper salt.
The term "pressure drop" as used herein means the difference between the pressure at the input end and the pressure at the output end of the converter. A high pressure drop is undesirable, because it reduces the efficiency of the engine. With a high pressure drop, as would be cauxed by an excessive amount of particulates in the converter, the engine must expend extra energy to force the exhaust gas through the converter, in addition to supplying energy for locomotion.
In the present invention, the particulates are not collected in a single compact layer through which the exhaust must pass, but instead the particulates are distributed throughout the entire converter so that the pressure drop builds up slowly.